Hydrogen peroxide as a mediator of 5-aminolevulinic acid-induced Na+ retention in roots for improving salt tolerance of strawberries

Abstract

To explore the mechanisms of 5-aminolevulinic acid (ALA)-improved plant salt tolerance, strawberries (Fragaria × ananassa Duch. cv. 'Benihoppe') were treated with 10 mg l^-1 ALA under 100 mmol l^-1 NaCl stress. We found that the amount of Na⁺ increased in the roots but decreased in the leaves. Laser scanning confocal microscopy (LSCM) observations showed that ALA-induced roots had more Na⁺ accumulation than NaCl alone. Measurement of the xylem sap revealed that ALA repressed Na⁺ concentrations to a large extent. The electron microprobe X-ray assay also confirmed ALA-induced Na⁺ retention in roots. qRT-PCR showed that ALA upregulated the gene expressions of SOS1 (encoding a plasma membrane Na⁺ /H⁺ antiporter), NHX1 (encoding a vacuolar Na⁺ /H⁺ antiporter) and HKT1 (encoding a protein of high-affinity K⁺ uptake), which are associated with Na⁺ exclusion in the roots, Na⁺ sequestration in vacuoles and Na⁺ unloading from the xylem vessels to the parenchyma cells, respectively. Furthermore, we found that ALA treatment reduced the H₂ O₂ content in the leaves but increased it in the roots. The exogenous H₂ O₂ promoted plant growth, increased root Na⁺ retention and stimulated the gene expressions of NHX1, SOS1 and HKT1. Diphenyleneiodonium (DPI), an inhibitor of H₂ O₂ generation, suppressed the effects of ALA or H₂ O₂ on Na⁺ retention, gene expressions and salt tolerance. Therefore, we propose that ALA induces H₂ O₂ accumulation in roots, which mediates Na⁺ transporter gene expression and more Na⁺ retention in roots, thereby improving plant salt tolerance.